The determination of the shape change of workpieces after forming processes is an important analysis to optimise the forming processes. In particular, strong, local loads of the formed material, for example if locally large forming processes occur, may be ascertained and optionally eliminated. Different methods are used to analyse the forming processes. For example, it is known to introduce structures, which are optically analysed before and after the forming, by applying gridlines, screens or dot screens of colour or corresponding etchings of the surface of the workpieces to be formed, so that the corresponding degrees of forming can be measured and shown. Moreover, it is known from the German patent specification DD 218 864 to print a varnish film with contours, shapes, measuring meshworks or figures on metallic or non-metallic material surfaces and to measure these after a corresponding forming of the workpiece. The increasing use of higher-strength or, respectively, highest-strength steels or steel alloys, for example to achieve weight savings, but also to achieve higher strengths, frequently requires the use of hot forming of the workpieces to provide necessary degrees of forming. During hot forming, the workpiece consisting of a steel or a steel alloy is heated to a hot forming temperature above 850° C. and formed in this state. During or immediately after the forming, the workpiece is greatly cooled, so that high-strength structures are produced. It has been found that during hot forming, the previously used possibilities for shape change analysis cannot be used. Owing to the high temperatures during the hot forming, neither the application of an electrolyte nor the application of previously provided colours are a possibility for allowing a shape change analysis, as the contrasts necessary for optical analysis of the shape change no longer existed in the previously known methods after hot forming.
The present invention is therefore based on the object of providing a method for determining shape changes of a workpiece made of steel or a steel alloy, which is also suitable for the shape change analysis of a workpiece after hot forming.